Purification of ferro-silicon alloys



United States Patent PURIFICATION OF FERRO-SILICON ALLOYS Gilbert S. Layne, James 0. Huml, and Walter H. Boeckler, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Feb. 6, 1967, Ser. No. 614,053

Int. Cl. C22c 33/00; C21c 7/04 U.S. Cl. 75-129 9 Claims ABSTRACT OF THE DISCLOSURE A process for purifying ferro-silicon alloys which comprises heating a ferro-silicon alloy, which is contaminated with other metal impurities, in the presence of a sulfurizing agent. The liquid mixture is allowed to come to equilibrium and cooled forming a metal sulfide slag phase containing the impurities and an essentially pure ferro-silicon phase.

BACKGROUND OF THE INVENTION The present invention relates to a method for the purification of ferro-silicon alloys and more particularly to a method of removing titanium, aluminum and other minor metal impurities from a ferro-silicon alloy.

Stringent impurity standards have been set for ferrosilicon alloys. The titanium level in ferro-silicon alloys is of special concern in the production of high purity silicon steels used for the demanding applications of silicon steel manufacturers. The principal impurities associated with ferrosilicon alloys are aluminum and titanium. Other minor impurities include calcium, magne sium, and manganese. The greater proportion of aluminum is removed from the ferro-silicon furnace product by various known gaseous treatments. However, hereto fore, adequate and economical techniques have not been developed to remove titanium and residual amounts of aluminum and other minor impurities from the furnace product. Likewise, no method has been developed for simultaneously removing both aluminum and titanium. As such, the only means known heretofore for controlling the titanium level in the final ferro-silicon product has been to select raw materials, consisting of silica ores and scrap iron, which have a low initial titanium content. This method is costly and does not always guarantee satisfactory results.

Methods have been suggested in the known art for producing substantially pure aluminum compounds from ferro-silicon-aluminum alloys but none teach a method for removing substantially all the aluminum simultaneously with titanium and other minor impurities from ferrosilicon alloys. To illustrate, United States Pat. No. 938,- 634, issued to Betts, alleges a method for producing essentially pure aluminum compounds. The process of this patent essentially comprises contacting a ferro-siliconaluminum alloy with an oxidizing, sulfurizing, or chloridizing reagent in an amount substantially less than is stoichiometrically required to remove all the aluminum from the alloy. The process thus allows the formation of aluminum compounds which are substantially free of compounds of more readily reducible metals, along with an impurity contaminated ferro-silicon alloy. The Betts process is therefore directed toward the production of pure aluminum.

By the method of the present novel process titanium, aluminum and other minor impurities now can be simultaneously and economically removed from a ferro-silicon furnace product.

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SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a novel method for simultaneously removing titanium, aluminum and other minor impurities from ferro-silicon alloys.

It is another object of the present invention to provide a novel process whereby impurities such as titanium and aluminum and the like may be removed from a ferrosilicon alloy after the ferro-silicon alloy has been prepared, thereby permitting the use of readily available low cost starting materials.

These and other advantages and objects will become readily apparent from a reading of the detailed description of the novel invention presented hereinafter.

The present novel process comprises heating an impure ferro-silicon alloy, e.g., an alloy contaminated with titanium, aluminum and the like, in the presence of a sulfurizing agent; allowing the system to come to equilibrium, thus forming a two-phase system of a separable, substantially impurity free ferro-silicon phase and a metal sulfide slag phase containing the impurities, and separating the essentially aluminum and titanium free ferro-silicon alloy from an impurity containing metal sulfide slag.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Usually in the practice of the present invention an impure ferro-silicon alloy, e.g., an alloy usually contaminated with titanium, alluminum and other like metal impurities, is heated sufficiently to liquefy the alloy and contacted with a sulfurizing agent. The system is maintained in the liquid state for a suflicient period of time and contacted with a sufficient quantity of a sulfurizing agent to convert the impurities present in the ferrosilicon starting material to their corresponding sulfides. The system is allowed to come to equilibrium whereupon, because of the face that sulfur and sulfur containing compounds do not transfer into the purified ferro-silicon alloy, two distinct and separable phases are created in the reaction mixture. One phase consists of a metal sulfide slag containing the sulfides of the impurities present in the original ferro-silicon starting material. The second phase consists of a substantially pure ferro-silicon alloy. The system is usually allowed to cool and the impurity-rich metal sulfide slag phase is separated from the substantially impurity free ferro-silicon alloy phase.

With the method of the present novel process it is possible to remove from impure ferro-silicon alloys substantially all the titanium and aluminum as well as other minor impurities such as calcium, manganese, magnesium and the like, generally associated with such alloys, and which usually are characterized by the fact that they form more thermodynamically stable sulfides than iron sulfide.

An inexpensive source of impure ferro-silicon alloy starting material is the residual alloy derived as a byproduct from the monohalide distillation process for the recovery of aluminum from the product derived in the thermocarbonaceous treatment of bauxite. Now, with the method of the present invention, the residual alloy can economically be processed to form valuable impurity free ferro-silicon alloys.

The sulfurizing agents employed in the operation of the present invention are characterized as being composed of sulfur or sulfur containing compounds which will readily react with the impurities present in the ferrosilicon alloy to form their corresponding sulfides. Usually sulfurizing agents which are employed in the method of the present invention consist of hydrogen sulfides or sulfur; metal sulfides having a metal cation, the metallic form of which volatilizes under the operating conditions of the present method; sulfides of iron and silicon, and

sulfides of titanium wherein titanium is in a high valence state. Specific examples of sulfurizing agents which may be employed are S, H S, FeS CdS, ZnS, TiS Ti S TiS, SiS SiS, and other like materials and mixtures thereof.

Some of the sulfurizing agents which may be used in the present process, such as, for example, naturally occurring pyrite, contain small amounts of oxygen containing compounds such as ferrous oxide and silicon oxide. The presence of these oxygen-containing compounds in the sulfide reactant does not detrimentally affect the operation of the present process. However, it is preferred that the sulfurizing agents contain no more oxygen-containing compounds than are stoichiometrically required to convert the aluminum impurity present in the initial impurity contaminated ferro-silicon alloy to .aluminum oxides. Generally, the pyrite materials contain such small quantities of oxide materials that their presence poses no problems to the operability of the present invention.

It is preferred that iron sulfide be present in the sulfide slag in the reaction mixture during the operation of the present invention. The iron sulfide containing slag induces a better extraction of the impurities out of the ferrosilicon phase into the metal sulfide slag phase.

The iron sulfide may be introduced into the system by a number of different methods. Iron sulfide may be introduced as a sulfurizing agent in the form of FeS (ferrous sulfide) or FeS (pyrite). When FeS is used, the impurities present in the system form their corresponding sulfides since their sulfides are usually more thermodynamically stable than is FeS. FeS is preferred since it is economical, allows the formation of the iron sulfide (FeS) and, as distinguished from using FeS as a sulfurizing agent, will not add appreciable quantities of iron to the ferro-silicon phase. When FeS is employed it is preferred that the operating conditions be arranged so that substantailly all the gaseous sulfur formed is utilized in the reaction system.

The requisite iron sulfide also may be introduced into the system by the use of a greater quantity of other sulfurizing agents than are stoichiometrically required to remove the impurities present in the ferro-silicon alloy. The iron in the ferro-silicon phase will form iron sulfides which will also be extracted into the metal sulfide slag phase with the sulfides of the impurities. It is appreciated that when the latter procedure is employed the terrosilicon phase will be depleted of iron and therefore the substantially pure ferro-silicon alloy products will have a greater proportion of silicon to iron than in the initial contaminated ferro-silicon alloy. Therefore, one of the advantages of the method of the present invention is the fact that not only can aluminum and titanium be simultaneously removed, but under proper conditions, impurity free ferro-silicon alloys with high proportions of silicon to iron may be formed.

A sufi'icient quantity of a sulfurizing agent must be employed to assure a substantially complete conversion of the impurities present in the ferro-silicon alloy to their corresponding sulfide compositions. The sulfurizing agent requirement can be predetermined by determining the impurity content of the initial impurity contaminated ferro-silicon alloy. Based on this determination, at a minimum, about the stoichiometric requirement of sulfurizing agent can be determined.

It has been found that it is desirable to operate with an excess of the sulfurizing agent. An excess of sulfurizing agent of from about percent to about 200 percent is preferred, however, greater amounts may also be employed.

As indicated hereinbefore, the molten ferro-silicon reaction mass must be maintained in a liquid state so as to produce a system comprising an immiscible liquid metal sulfide slag and a molten ferro-silicon alloy. The operable temperature range to be employed, therefore, will vary accordingly with the constituents present in the system. The minimum operating temperature is the fusion temperature of the highest melting phase present in the system. For example, in a system constituting an iron sulfide slag, preferably FeS and a ferro-silicon-titaninmaluminum alloy the minimum temperature is about 1300 C. The maximum operable temperature is the temperature at which the silicon present in the system forms gaseous silicon-sulfide compositions and is dependent on the concentration of silicon in the reaction mass. As the silicon concentration increases, the maximum operating temperature decreases. To illustrate, at atmospheric pressures a temperature range of from about 1300 C. to about 1600" C. is appropriate to produce a ferro-silicon alloy comprising approximately 35 weight percent of silicon. Because of the volatility of silicon, ferro-silicon compositions comprising at a maximum about 50 percent silicon are operable in the practice of the present invention. The sulfide forming reaction is exothermic, therefore the temperature requirements are in part achieved by the reaction itself.

The rrnethod of the present invention may be operated at or near atmospheric pressure and it has been observed that it is desirable to operate the process under a nonreactive atmosphere, such as argon. Pressures greater than atmosphere are desirable in that they help suppress the volatilization of silicon sulfides, particularly at higher operating temperatures and also allows the formation of ferro-silicon alloys with greater proportions of silicon. By operating the present process at about 200 pounds per square inch of pressure the silicon concentration in the product alloy may be increased by about 10 Weight percent over the maximum silicon concentration when the process is operated at normal pressures. Therefore, it is preferable to operate under elevated pressures when high concentations of silicon are desired in the final product, e.g. greater than about 50 weight percent of silicon in the final ferro-silircon alloy product.

The reaction times involved in the operation of the present invention are quite short and contact times of from about one-half hour to about four hours 'have proven sufficient to substantially convert the impurities to their corresponding sulfides and to form the separable phases. Stirring or agitation of the liquid system promotes the reaction and decreases the reaction time. Likewise, lancing the liquid system with gaseous H 8 or S or the introduction of free gaseous sulfur forming compounds such as, for example, FeS or SiS will also promote the reaction and reduce the reaction time.

The process of the present invention as described herein in detail is further illustrated by the following examples.

Example 1 An impure ferro-silicon alloy containing approximately 65 Weight percent Fe, 20 weight percent Si, 10 Weight percent Ti, and 5 weight percent Al was provided. To 2380 grams of this composition was added 2125 grams of ferrous sulfide (FeS) and the reaction mass was heated to about 1450 C. in a graphite crucible under an atmospheric pressure of argon whereupon a system of immiscible liquid slag and molten ferro-silicon-aluminumtitanium composition was produced. The system was maintained at the indicated temperature for about 4 hours and then cooled. The products produced were an aluminum and titanium depleted ferro-silicon phase and a metal sulfire slag phase. An analysis of these phases showed the following constituent dispositions.

An alloy containing approximately 47 weight percent Fe, 33 weight percent Si, 10 weight percent Ti and 10 Percent Fe Al Ti Si S Slag phase 30. 12. 1 15.0 .002 Balance Ferro-silicon phase... 78. 1 0. 6 0. 5 21. 2 O. 1

In a manner similar to the foregoing examples other sulfurizing agents may be substituted for ferrous sulfide and impure ferro-silicon alloys can be treated to remove titanium and aluminum as Well as other impurities. As indicated before, the method of the present invention is especially suited for treating residual alloys obtained from the thermocarbonaceous treatment of bauxite and subsequent extraction of aluminum by the monohalide distillation process.

Various modifications may be made in the method of the present invention without departing from the scope or spirit thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A process for separating titanium impurities from ferro-silicon alloys which comprises:

(a) reacting a fused ferro-silicon alloy containing at least titanium as an impurity with a sulfurizing agent, said sulfurizing agent being reactive with the metal impurities and titanium in the alloy and forming sulfides with the metal impurities and titanium said sulfurizing agent being provided in an amount at least stoichiometrically required for reaction with substantially all said metal impurities and titanium in said alloy, for a time sufficient for said reaction to be substantially complete to form a two-phase system of impurity depleted ferro-silicon alloy and an impurity rich metal sulfide slag, and

(b) separating said impurity free ferro-silicon alloy from said metal sulfide slag.

2. The process as defined in claim 1 wherein said sulfurizing agent is provided in an amount ranging from about 10 weight percent to 200 weight percent in excess of that stoichiometrically required to convert substantially all of the metal impurities and titanium to their corresponding sulfides.

3. The process as defined in claim 1 where said ferrosilicon alloy contains up to weight percent of silicon.

4. The process as defined in claim 1 where said treatment temperature ranges from about 1300 to 1600 C.

5. The process as defined in claim 1 wherein said sulfurizing agent is selected from the group consisting of hydrogen sulfide, sulfur, metal sulfides having a metal cation, the metallic form of which volatilizes under the operating conditions of the present process; and iron sulfides, silicon sulfides, titanium sulfides and mixtures thereof.

6. The process as defined in claim 1 wherein the sulfurizing agent is FeS 7. The process as defined in claim 1 wherein the reaction is conducted while under elevated pressures.

8. The process as defined in claim 1 wherein said sulfurizing agent is provided in an amount in excess of that stoichiometrically required for reaction with substantially all the metal impurities and titanium in said ferro-silicon alloy.

9. The process as defined in claim 1 wherein the sulfide slag contains iron sulfide.

References Cited UNITED STATES PATENTS 855,157 5/1907 Becket 10 1,415,516 5/1922 Bridge 7527 1,982,959 12/1934 Kuhlmann 7563 2,301,360 11/1942 Brennan 75-63 3,396,012 8/1968 Huml 7563 L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 7558, 59 

